The trickiest part of fighting Onyxia is during her air phase. A DK has only two truly ranged attacks - Icy Touch and Death Coil. Outbreak (ranged but doesn't do direct damage) is helpful for getting the Blood Plague DoT applied as well as http://www.wowdb.com/spells/59921-frost-fever, but it's harder to do because Onyxia is only in range of it when she's near a wall. Do your best to keep Frost Fever ticking on the dragon, but be mindful of your available runes since you will be using Death Strike a lot.

Use your DPS cool-downs at or shortly after initiating the fight. It's best to push her as low as possible before she gets into the air.

Kill the wyrmkin soldiers but leave the whelplings alive. Kill whelplings as needed to maintain your DS absorb and gain runic power. Use the runic power to cast Death Coil at the boss.

Follow Onyxia around as best you can, using Anti-Magic Shell when off CD to absorb her fireball and gain more runic regeneration.

Kill the adds that remain just before or as she lands back on the ground. Particularly important if you don't have an item or ability (you DO have Lichborne as a talent, right...?) that breaks fear.